I would try lowering light intensity way down and having 6-8 hour nights for a few days. Perhaps it will help. Perhaps it won't. Easy enough to try in veg anyway.
Umol/s tells you how many photons it produces in a second. Same thing as ppf (photosynthetic photon flux).
Umol/j tells you how many photons it produces per joule/watt. Efficiency.
PPFD is your light output per area (photosynthetic photon density).
Yeah, third node is a good place to look when the plant has about seven nodes. Might take a little longer than that, but seven nodes is when I start looking for preflowers on vegging plants.
This is my high tech cloning system with full spectrum led lighting. I took these three cuts at about three weeks of 12/12 and the slowest one took almost three weeks to root, but first had roots in a week.
If you're in europe this would be a great choice. There's a ppfd map for a 60*60cm space on the site. Not as fancy looking as some, but the plants won't care.
https://www.led-tech.de/en/ZEUS-LIGHTNING-Basic-240W-plant-light-3500k
I do it so there can be some air flow and light penetration in my bushy little plants. I'm still experimenting with how many tops is optimal and may go for a lower count so the need for defoliation will be less, but as it is I would have a wet mess if I didn't defoliate at all. Leaves that don't...
If I was having difficulties with keeping temps up I would try using my lung room to raise temp. Less air exchange in the vent room should help. Or I could even have my tent in a tent. The leds do produce loads of heat, but we suck it out of the grow area and send it on it's way.
The inverse square thing is because the light spreads to a larger area thinning out the photon density. Photons exist as long as they are not absorbed... They will literally travel forever in space if they don't hit anything. In a grow tent the walls and plants are only things absorbing photons...
^ Yeah, water filtration will solve that problem. If you have a bong you can even try it out with a volcano bag to see if it helps before buying anything.
This is hps, right?
If your space and temperature allows it stick 600w. If too hot or too much light then you can drop to lower levels. Super lumens just adds a few watts more and shortens bulb life. It's a gimmick imo and I never used it.